Physics past paper question - hard disks

[jsmith613]

Homework Statement

I have attached the question

Homework Equations

The Attempt at a Solution

so the first thing I did was find the frequency = 120 Hz
The next thing I did was find the circumference = 0.2796 m (from pi*d)

Now I know that the answer is no. magnets per disk * 120
BUT I am told the LENGTH of a magnet not the width (I know exacly what I have to do - circumerence / length of magnet = no. magnet per disk) but I cannot see why this works (I have the MS so I know I had to do that)

Please could someone explain why this is the case

thanks

 

[jsmith613]

well it seems to me actually this is the ONLY way to look at it...I will put the issue down to semantics.

never mind :)

 

[gneill]

The "length occupied by each separate magnetized region" is its length along the circumference of the circle that the head is reading.

I suspect that you were thinking of the length corresponding to the radial lengths of the magnets depicted in the diagram (since clearly they show the magnets being longer in the radial direction than the circumferential direction), so you were thinking that what you really needed was the widths of those magnets. In this case it may be that the given diagram has caused some confusion due to assumptions it led you to make on the geometry being described in the problem itself.